Here is the deal ... the country star is being sued by a woman who claims she was his wife Trisha Yearwood's makeup and hairstylist for years ... before he allegedly raped her in a hotel room in 2019.

GB responded to the allegation Thursday not long after news of her lawsuit broke ... saying he's been "hassled to no end with threats, lies, and tragic tales of what my future would be if I did not write a check for many millions of dollars.  It has been like having a loaded gun waved in my face."

In the docs, the woman claims Garth held her 5-foot-frame by her ankles and dangled her upside down as he penetrated her vagina, slamming his penis inside her so hard she felt as if she were breaking in two.

She claims after the rape it was business as usual, and she did Garth's hair and makeup for the event. She says she was dealing with financial hardships and needed the work, and Garth preyed on her financial situation.

Garth filed his own lawsuit against the woman about a month ago ... accusing her of "extortion and defamation of character" ... which at the time was filed anonymously "for the sake of families on both sides."

The lawsuit from GB makes clear the accuser allegedly knew she could inflict a great deal of harm to Garth's career and reputation if she followed through with her allegations ... which she allegedly planned to do sans a massive payday from the millionaire.